In microscopic examinations of objects, a resolution which is as high as possible or a high space-bandwidth product and/or a high contrast is/are desirable for many applications. Techniques which allow a comparatively high-resolution result image to be obtained by combining a plurality of images with a comparatively low resolution in the spatial frequency space (k-space) are known. Fourier ptychography is such a technique; see G. Zheng et al., in Nature Photonics 7 (2013) 739-745. In this method, an object to be examined by microscopy is successively illuminated in a partially coherent manner under different illumination directions. For each illumination direction, the object illuminated thus is imaged and recorded using a microscope and the image captured thus is stored (image capture). A stack of images is obtained in this way, with each image having a different illumination direction assigned thereto and the images having a comparatively low resolution. A phase and amplitude distribution of the object is then calculated (image evaluation) using this stack of images by means of an iterative algorithm, for example an error reduction algorithm, a hybrid input-output algorithm and/or a Gerchberg-Saxton algorithm. Said distribution may be stored and depicted as a result image, which has a comparatively high resolution. Here, a spectrum of the object is reconstructed by the algorithm instead of the object itself, and so the phase and amplitude distribution of the object may be obtained by a further Fourier transform.
In a Fourier ptychography device used in the aforementioned publication by G. Zheng et al., the various illumination directions are realized by a matrix-shaped light-emitting diode array or a light-emitting diode (LED) array as an illumination device below the object, with a microscope then imaging the object in transmission. The working distance between the LED array and the object is selected in such a way in the process that each individual LED has a sufficiently high degree of coherence. The images may be captured with different illumination directions by switching on individual LEDs in sequence.
However, this procedure for illumination has various disadvantages. Thus, the illumination of the object only has restricted homogeneity in certain illumination directions, particularly if the illumination directions are very different. Strongly deflected illumination directions by LEDs at the edge of the LED array typically correspond to a greater distance between the LED and the object and therefore lead to a reduced light power. Moreover, an installation space required for such an LED array and a required distance between the LED array and the object are relatively large. Finally, such an illumination technique cannot be easily integrated into existing illumination concepts of conventional microscopes. In particular, it cannot be used as an additional module, by means of which it is possible to switch between conventional incoherent illumination and coherent illumination from various illumination directions.
In general, an imperfection of the illumination device—such as e.g. restricted coherence of the light, inhomogeneous illumination, only restricted selection of illumination directions, other aberrations of illumination fields, etc.—may have a negative effect on the quality of the result image when Fourier ptychography techniques are applied. However, it may often be necessary to accept such restrictions in the quality of the illumination device in order to obtain a practically implementable illumination device. Then, it may once again not be possible, or only possible to restricted extent, to use the conventional Fourier ptychography techniques for image evaluation purposes. In particular, a reduction in the result image quality may occur.
Therefore, there is a need for improved techniques for determining a result image on the basis of Fourier ptychography. In particular, there is a need for those techniques which facilitate a comparatively high quality of the result image, even in the case of imperfections in the illumination device. There is a need for those techniques for determining a result image on the basis of Fourier ptychography which facilitate reliable and good results when applied in very different microscopy devices. There is also a need for those techniques which facilitate a comparatively simple illumination device and/or an illumination device with little structural complexity in conjunction with Fourier ptychography techniques.